Are you leaving the paper on the back until you join the blocks together?
It seems most PPers like to keep the paper on till the seams are sewn together. I on the other hand I like to remove the paper before I sew the seams together. IMHO the bulk of all that paper and fabric is very thick and the feed dogs don't grab the paper as you sew and slipping occurs. There is also no paper in the seams to remove making the job easier.
Seams just fit toghther better. If there is some bias involved which there usualy is, I use a bit of starch and press after removing the paper. Hope this helps a little.
peace